1. FIELD OF THE INVENTION
The invention relates to a method for the continuous electrolytic plating of a metal strip with a metallic plating layer in which the strip is continuously passed as cathode through an electrolysis cell having an anode, the plating layer being deposited on the strip from an electrolyte between the strip and the anode.
In the following the invention will be discussed with particular reference to the manufacture of tin plate, but is not restricted to this application and is also applicable to other plating processes, such as electroplating with other metals such as zinc, cadmium, chromium, nickel, lead, gold or with alloys of these and electroplating with co-deposits such as zinc with aluminum oxide for example.
2. DESCRIPTION OF THE PRIOR ART
In the manufacture of tin plate by continuous electrolysis, a known phenomenon occurring in practice is that at particular locations on the strip surface more tin is deposited than at other places. In order to overcome this defect, which can lead to rejection of the tin plate on grounds of quality, , an additive, a so-called brightener, is included in the electrolyte, which influences the formation of crystallization nuclei (so called nucleation) and their growth. It is generally assumed that the deposition process at particular active locations on the surface of the strip, where more crystallization nuclei are formed which grow more rapidly, is delayed by the brightener, so that an evenly distributed plating layer is achieved. However, the brightener causes the charge transfer reaction to become slower, so that the energy consumption increases.
In practice this occurrence of non-uniform plating is observed after the tin plate has been manufactured. The tin plate manufactured subsequently in the same plating line therefore has the same defect, and may also need to be rejected.
Some attempts to monitor the formation of a plating layer, for various purposes, are described in the prior art.
In U.S. application Pat. No. 2,895,888, in a continuous plating process, it is proposed to measure final plating thickness of tin or zinc on strip by means of a nuclear beta radiation reflection gauging system located after the electrolysis cell. The reflected beta radiation is detected and used to calculate the plating layer thickness which is compared with a desired thickness. From this comparison, adjustment of the electrolysis current is made.
U.S. application Pat. No. 3,827,963 describes a batch process of electrolytic plating in which the progress of the layer formation is watched using a light beam directed at the work-piece being plated throughout the plating process. Light reflected from the work-piece is measured and compared with a target value. The target value may represent the desired end of plating, or a desired point at which the process is switched from current mode to halo mode.
Similarly, DE-A-No. 2,312,578 described an anodizing batch process in which a reflected light beam is used to monitor progress of the layer formation, e.g. to determine when the desired layer is formed.